The present invention relates in general to communication systems, and in particular to methods and systems for improving various aspects of communication systems utilizing multi-carrier transmission techniques such as orthogonal frequency division multiplexing.
Wireless personal communication devices have proliferated over the past several years. Integration of more functionality such as multimedia capabilities into these devices has created an ever increasing demand for enhanced broadband communication methodologies. Unlike satellite communication where there is a single direct path from a transmitter to a receiver, personal wireless communication devices must operate in a multi-path environment. Multi-path propagation is caused by the transmitted signal reflecting off of objects such as buildings, automobiles, trees, etc., that may be encountered along the signal path. This results in the receiver receiving multiple copies of the transmitted signal each having different delay, attenuation and phase shift depending on the length of the path and the material composition of the objects along the path. The interference between the multiple versions of the transmitted signal, referred to as inter-symbol interference (ISI), is a common problem that can severely distort the received signal.
Orthogonal frequency division multiplexing (OFDM) is one type of multi-carrier data transmission technique that has had some success in addressing ISI, distortion and other problems associated with multi-path environments. OFDM divides the available spectrum into multiple carriers, each one being modulated by a low rate data stream. Multiple user access is achieved by subdividing the available bandwidth into multiple channels, that are then allocated to users. The orthogonality of the carriers refers to the fact that each carrier has an integer number of cycles over a symbol period. Due to this, the spectrum of each carrier has a zero at the center frequency of each of the other carriers in the system. This results in no interference between the carriers, allowing them to be spaced as close as theoretically possible. Each carrier in an OFDM signal has a very narrow bandwidth, thus the resulting symbol rate is low. This results in the signal having a high tolerance to multi-path delay spread, as the delay spread must be very long to cause significant inter-symbol interference. Coded orthogonal frequency division multiplexing (COFDM) is the same as OFDM except that forward error correction is applied to the signal before transmission. This is to overcome errors in the transmission due to lost carriers from frequency selective fading, channel noise and other propagation effects. In the description presented herein, the terms OFDM and COFDM are used interchangeably.
In OFDM the sub-carrier pulse used for transmission is chosen to be rectangular. This allows the task of pulse forming and modulation to be performed by an inverse discrete Fourier transform (IDFT). IDFT is implemented very efficiently as an inverse fast Fourier transform (IFFT) which would then require only an FFT at the receiver end to reverse the process. In order to accurately reproduce the transmitted signal, the receiver must estimate and compensate for various distortions in the received signal, including channel transfer function, carrier frequency offset and clock frequency offset.
The channel transfer function reflects the effect of the propagation channel on a transmitted signal, including multipath delay effects. A number of techniques are known for estimating and compensating for the channel. Channel estimation may be performed in the receiver either in the time domain (i.e., before the receiver performs the FFT on the received signal) or in the frequency domain (i.e., after the receiver performs the FFT on the received signal). Compensation for the channel using the channel estimate is generally performed in the frequency domain.
Carrier frequency offset (CFO) arises from the finite tolerance of RF components used in the transmitter and receiver. Even though the transmitter frequency is usually known to the receiver, due to RF tolerance, the frequency at which the receiver operates may not match exactly. This offset causes a phase shift in the received signal. Additionally, in systems where the transmitter or receiver is mobile, Doppler effects may also give rise to carrier frequency offsets. In order to provide accurate reproduction of the transmitted signal, this CFO-induced phase shift must be estimated and compensated by the receiver.
Clock frequency offset (CLO) arises from a mismatch between the sampling rate at the receiver and the sampling rate at the transmitter; again, the mismatch causes a phase shift in the received signal that must be estimated and compensated by the receiver. In addition, CLO may cause synchronization errors if the receiver assigns too many or too few samples to a symbol. Some OFDM standards, such as the 802.11a standard, reduce the complexity of the clock and carrier frequency offset problem by requiring that the carrier frequency and clock frequency in the receiver be derived from the same oscillator. But these standards do not eliminate the problem since the frequency of the receiver's oscillator will in general not exactly match the frequency of a transmitter's oscillator.
A number of methods for estimating phase shifts caused by CFO and/or CLO have been proposed. Some of these methods operate on received signals in the time domain (i.e., before the FFT is performed by the receiver); these include computations based on time correlations. Others operate in the frequency domain (i.e., after the FFT is performed by the receiver) and may involve least squares or maximum likelihood computations.
Existing methods for estimating phase shifts assume that the only noise present is additive white Gaussian noise, i.e., that the noise power spectrum is independent of carrier frequency. In many implementations of OFDM, however, non-white noise (i.e., noise with a power spectrum that depends on frequency, such as 1/f noise) may be present due to the receiver front end electronics or other noise sources. This non-white noise adversely affects the existing phase estimation methods, leading to increased error rates. Thus, a need exists for a method of computing carrier frequency and clock frequency offsets that does not rely on the assumption of white noise.